The present invention relates generally to fuel sensors for vehicles and, more particularly, to a ceramic resistor card assembly for a fuel sensor of a vehicle.
It is known to provide a fuel sensor in a fuel tank of a vehicle to sense or indicate a level of fuel in the fuel tank. Typically, the fuel sensor includes a ceramic resistor card operatively connected to structure for a fuel module and a wiper assembly pivotally connected to the structure for engaging the resistor card. The wiper assembly has one end connected to a float to rotate the wiper assembly relative to the resistor card based on a level of fuel in the fuel tank. The wiper assembly has another end with either single or multiple contact loops extending toward the resistor card and contacts on the contact loops to engage conductive inks on the resistor card. Examples of such fuel sensors are disclosed in U.S. Pat. No. 5,746,088 and U.S. Pat. No. 6,021,668.
Typically, the resistor card is manufactured by using a ceramic substrate, which is screened (printed) with conductive and resistive inks, which are subsequently fired at high temperatures in a kiln. Conductive inks used in the manufacturing of the ceramic card contain various metals and binders. Metals used in the conductive inks include Silver, Platinum, Palladium, Gold, Copper, as well as others. All conductive inks used in these fuel sensors contain Silver for its conductive properties and the ease with which electrical wires can be soldered to the substrate. Other metals such as Palladium are used to provide strength against shear forces exerted by the sliding contacts.
Ink strength is frequently expressed as a ratio of Silver to Palladium. Common ranges are in the 6:1 to 2:1 range. While Silver has many desirable properties as a conductor, it is a metal that is chemically very reactive to compounds frequently found in fuels. Sulfur is but one of these substances. When chemically attacked, the silver forms various compounds such as Silver Sulfide (AgS), which are non conductive. These deposits of nonconductive material generate contact resistance and create shifts and/or spikes in the output signal of a rheostat or potentiometer. This contact resistance can also appear as signal xe2x80x9cnoisexe2x80x9d in the output of the ceramic resistor card. Signal xe2x80x9cnoisexe2x80x9d are readings where the output xe2x80x9cspikesxe2x80x9d momentarily from what the output should be. Either condition may result in inaccurate fuel level indications, which are undesired.
The firing process (kiln) used to bond the inks to the substrate of the ceramic resistor card creates Oxides of the various metals on the surface of the conductive ink. These Oxides protect the ink from chemical attack by compounds such as the sulfur, since the metals have already xe2x80x9creactedxe2x80x9d with the Oxygen. However, the back and forth movement of the contacts on the wiper assembly across the conductive ink creates mechanical abrasion of the contacts and the conductive ink surface referred to as xe2x80x9cburnishingxe2x80x9d. Once the Oxide layer has been rubbed through, the Silver is open to attack by Sulfur and other reactive components found in hydrocarbon fuels.
Recently, it has been proposed to have a ceramic resistor card including conductive inks utilizing Gold (Au), Platinum (Pt), or a combination of the two (2) metals. The shear strength of Gold (Au) is so low as to allow smearing and trenching of the circuit segments due to the interaction of the sliding contacts. Gold (Au) also has a tendency to adhere to contact and then redeposit in a different location.
Platinum (Pt) is an excellent catalyst. As Benzene elements in the fuel are placed in shear by the sliding action of the contacts in the presence of Platinum (Pt), there is a tendency to xe2x80x9cripxe2x80x9d atoms out of the Benzene ring. The resulting substance is lacquer. When lacquer (an organic nonconductor) builds up on the contact surfaces, the resulting contact resistance has the same effect as the Silver sulfide compounds.
While a combination of Gold (Au) and Platinum (Pt) creates a harder (stronger) wear surface, it is desirable to provide better durability to the ceramic resistor card. It is desirable to provide a ceramic resistor card assembly for a fuel sensor that is void of Silver or has a very low content of Silver in the conductive ink. It is also desirable to provide ceramic resistor card assembly for a fuel sensor that incorporates inks that are more resistant to chemical attack than those inks commonly used. It is further desirable to provide a ceramic resistor card assembly for a fuel sensor that is impervious to attack by sulfur compounds.
It is, therefore, one object of the present invention to provide a new ceramic resistor card assembly for a fuel sensor for a fuel tank.
It is another object of the present invention to provide a ceramic resistor card assembly for a fuel sensor incorporating a conductive ink that results in a harder and more chemically resistant wear surface.
To achieve the foregoing objects, the present invention is a ceramic resistor card assembly for a fuel sensor including a card portion made of a ceramic material. The ceramic resistor card assembly also includes a first conductive ink disposed on the card portion to form a fuel level portion. The ceramic resistor card assembly further includes a second conductive ink disposed on the card portion to form at least one contact for the fuel level portion. The first conductive ink is void of Silver and the second conductive ink contains Silver.
One advantage of the present invention is that a ceramic resistor card assembly is provided for a fuel sensor in a fuel tank of a vehicle. Another advantage of the present invention is that the ceramic resistor card assembly has a conductive ink path either void of silver or a very low content of silver. Yet another advantage of the present invention is that the ceramic resistor card assembly is impervious to attack by sulfur, chlorine, or other chemically active compounds. Still another advantage of the present invention is that the ceramic resistor card assembly provides a surface on which the existing wiper contacts ride, which contains no silver, improving performance. Yet a further advantage of the present invention is that the ceramic resistor card assembly has Palladium (Pd) added to a conductive ink made of an alloy Gold (Au) and Platinum (Pt) in the contact area to provide a better chemically resistant wear surface. Still a further advantage of the present invention is that the ceramic resistor card assembly has a conductive ink made of an alloy of Palladium (Pd) and Gold (Au) and Platinum (Pt) in the contact area, providing a minimal cost impact.
Other objects, features, and advantages of the present invention will be readily appreciated, as the same becomes better understood, after reading the subsequent description taken in conjunction with the accompanying drawings.